The effect of alkane tail length of C_iE₈ surfactants on transport to the silicone oil-water interface

Detailed surfactant transport studies have typically been restricted to the air-water interface. This is mainly due to the lack of experimental devices and techniques available to study liquid-liquid interfaces. As a result, there is a lack of relevant data and understanding of surfactant behavior in microfluidic studies and emulsion applications. Using a novel shape fitting algorithm for a pendant drop capable of handling fluids of similar densities, i.e. low Bond numbers, we measure the dynamic surface tension as a function of bulk surfactant concentration at the silicone oil-water interface for a homologous series of C_iE₈ nonionic surfactants. We show that the isotherms governing equilibrium at the oil-water and air-water interfaces are very different. Using a scaling analysis comparing two governing mass transport timescales, we demonstrate that there exists a transition from diffusion-limited to kinetic-limited dynamics at the silicone oil-water interface. Adsorption rate constants are determined from a one parameter nonlinear fit to dynamic surface tension data. These results demonstrate that the dynamics of interfacial transport are highly dependent on the immiscible fluids that form the interface.